The present invention relates to a method for controlling the operating range of a pulse modulator. The modulator produces modulated output radiation, for example, in the optical range, from input radiation as a function of a control signal.
Stable pulse sources are required to produce pulses in optical message transmission networks. One simple and cost-effective method for producing pulses from a continuous-wave source via high-speed optical modulators is described in DE 199 24 347.6. However, the long term stability of the pulse source is problematic in this method. In order to avoid operating range changes, inherently stable modulators have, until now, been used for low data rates, in which the long term stability is achieved by complex design measures. The same problems also occur in data modulators.
One object of the invention is, therefore, to provide a simple method for controlling the operating range of a modulator, which ensures that the operating range of the modulator is fixed. Furthermore, an associated drive unit is intended to be specified.
The present invention is based on the knowledge that the operating range of the modulator close to the operating point is a major operating parameter. The operating range varies for various reasons, for example, as a function of the operating temperature, or due to aging of the modulator. Furthermore, the operating range is subject to scatter due to production tolerances between different modulators.
Preferably, according to the present invention, the modulator produces modulated output radiation from input radiation as a function of a control signal. The actual operating range of the modulator with respect to its transmission characteristic depends on the amplitude of the control signal. Starting from the output radiation, the mean radiation power is detected in at least one predetermined frequency range. The mean radiation power is the radiation power averaged over the frequencies. Cyclic deflection of the operating range is forced to occur at the deflection frequency. A control signal for controlling the operating range is produced as a function of the deflection of the operating range. The amplitude of the control signal is varied as a function of the control signal, so that the discrepancy between the actual operating range and the nominal operating range becomes less.
The operating range can be controlled very accurately and in a simple manner by detecting the radiation power and using the power as a control variable. Changes to the transmission characteristic can also be included in the control system, by indirect reference to the transmission characteristic. This procedure allows the control process to be carried out even without presetting any nominal power.
In one embodiment, the derivative of the function of the operating range and detected power is used as a control variable. The control process refers to a point in the function at which the derivative has the value zero. A minimum, a maximum, a point of inflection or some other point at which a derivative has the value zero in the power curve is chosen, by way of example, as the reference point. Thus, using the reference point, the control loop controls to this point without any additional tuning.
Known method of control engineering (e.g., analog or digital control methods based on the use of proportional, integral and differential regulators, and combinations of them) can be used for control. However, very good control loops result if the control variable is detected using phase-sensitive detection, which is also known as the lock-in method. Phase-sensitive detection has the advantage that the control process can be carried out comparatively independently of disturbance variables (e.g., signal noise). Phase-sensitive detection is further explained in the book xe2x80x9cElectronic Measurement and Instrumentationxe2x80x9d, Klaas B. Klaassen, Cambridge University Press, 1996, pages 204 to 210.
Preferably, the modulator is either a pulse modulator, which is driven at a drive frequency predetermined by a cyclic control signal, or a data modulator, which is driven by a control signal dependent on the data to be transmitted, with half the data rate being referred to as the drive frequency.
In one embodiment of the present invention, the frequency range, which is predetermined for controlling the operating range, includes all the output radiation frequencies which can be detected by a transducer unit. There is no need for any filters for selecting a frequency range.
In yet another embodiment, the frequency range, which is predetermined for controlling the operating range, includes only a portion of the output radiation frequencies detected by a transducer unit. Although this necessitates filter units being connected downstream from the transducer unit, it creates additional degrees of freedom in the choice of the control variable.
In one embodiment, in which only some of the frequencies detected by the transducer unit are used, the predetermined frequency range includes twice the drive frequency. The predetermined frequency range does not include other multiples of the drive frequency and the drive frequency itself. Thus, if there are any discrepancies in the actual operating range and the nominal operating range, the radiation power in the vicinity of twice the drive frequency falls considerably, so that the power in this region is highly suitable for control purposes. The frequency range used has a width of 0.3 times twice the drive frequency, in one embodiment.
In embodiments with predetermined frequency ranges in the radio-frequency band, and a nominal operating range of the modulator which is symmetrical about a transmission maximum (Return to Zero (RZ) operation), or which is about a transmission minimum (carrier-suppressed RZ operation), the amplitude value of the control signal is controlled via a control loop. Without being tuned, the control loop is aligned with a control point at which the mean radiation power is maximized within the frequency range predetermined for controlling the operating range.
In yet another embodiment, the frequency range, which is predetermined for controlling the operating range, includes only frequencies which are well below the drive frequency (i.e., the frequency is low in comparison to the drive frequency). For example, these frequencies are less than one-tenth of the drive frequency. Thus, components with low-cut-off frequencies may be used, even if the drive frequency is in the radio-frequency band.
If, in one embodiment, the nominal operating range of the modulator, preferably of a pulse modulator, in the low-frequency range is symmetrical about a transmission minimum (carrier-suppressed RZ operation) or is symmetrical about an operating point between a transmission point of inflection and a transmission maximum (clock RZ operation), then the amplitude value of the control signal is controlled via a control loop which is aligned, without tuning, to a control point at which the mean radiation power is a maximum within the frequency range predetermined for controlling the operating range.
If, in another embodiment, the nominal operating range of the modulator, preferably of a pulse modulator, in the low-frequency band is, in contrast, symmetrical about a transmission maximum (RZ operation) or is symmetrical about an operating point between a transmission minimum and a transmission point of inflection (clock RZ operation), then the amplitude value of the control signal is controlled via a control loop which is aligned, without tuning, to a control point at which the mean radiation power is a minimum within the frequency range predetermined for controlling the operating range.
The control loops for controlling the operating range are tuned, in other embodiments, meaning the control loop is controlled to a point close to the control point, in the nominal operating range of the modulator. The control loop can be tuned by known methods of control engineering (e.g., by deliberately applying a disturbance variable).
The amplitude value of the control signal can, for example, easily be varied by adjusting the gain of an amplifier at whose output the control signal is produced. A clock signal (e.g., a sinusoidal or square-wave signal) or a data signal is applied to the input of the amplifier.
A control variable with the correct mathematical sign can easily be obtained if a small discrepancy is forced to occur between the operating range and the nominal operating range, for control purposes. The power is detected for at least two operating ranges.
The control variable with the correct mathematical sign is then derived from the detected power. One method which operates with forced discrepancies in the operating range is phase-sensitive detection. Phase-sensitive detection is also referred to as the lock-in method, see Klaas B. Klaassen, xe2x80x9cElectronic Measurement and Instrumentationxe2x80x9d Cambridge University Press, 1996, pages 204 to 210.
In one embodiment, the discrepancy in the operating range is forced via a periodic deflection signal at a predetermined deflection frequency. The deflection signal is additively or subtractively superimposed on the control signal. A signal which is dependent on the detected power is multiplied by a cyclic reference signal, whose frequency matches the deflection frequency. A signal resulting from the multiplication is used, after low-pass filtering and preferably after subsequent integration, for varying the amplitude of the control signal. The cut-off frequency of the low-pass filter governs the response time of the control loop which is, for example, between 10 and 100 milliseconds. The refinement is based on the knowledge that the signal value of the DC component that passes through the low-pass filter is a measure of the first derivative of the power function.
The deflection signal has a cosine or sinusoidal waveform. However, other deflection signals (e.g., a square waveform) are also used. If the reference signal frequency corresponds to a multiple of the deflection frequency, then it is possible to detect points at which higher derivatives are zero, for instance, a point of inflection at twice the deflection frequency.
The operating point can be controlled in a similar way at the same time. The deflection frequency for controlling the operating point and the deflection frequency for controlling the operating range are chosen appropriately. Deflection frequencies that differ from one another are thus used, for example a deflection frequency of 3 kHz and a deflection frequency of 5 kHz.
In a pulse modulator or a data modulator, the input radiation is produced via a continuous wave light source or via a pulsed light source.
In further embodiments, the drive frequency and the data rate are more than 1 Gigahertz or 1 GBit/s, respectively, preferably 5 Gigahertz and 5 GBit/s, respectively. In yet another embodiment, the modulator operates in the optical band. A modulator which contains a Mach-Zehnder interferometer is suitable. The modulator""s transmission characteristic is, for example, in cosine form, or sinusoidal. However, modulators with other transmission characteristics are also used.
The present invention also relates to a drive unit for carrying out the methods mentioned above. The technical effects cited for the method also apply to the drive unit and to its embodiments.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.